1. Field of the Invention
This invention relates to a dual computer system including a pair of computer systems for carrying out the same operation, a dual system controller for monitoring the results of the operations, by the computer systems and applying the results of the operations to an external output register upon confirmation of coincidence between the results of the operations, and means for running a diagnose routine through the computer systems when there is non-coincidence between the results the operations or when an abnormal condition occurs in the dual system controller, so that the dual computer system can be placed in one of a plurality of operating modes including parallel operation of the computer systems with the first system working as the main system, parallel operation of the computer systems with the second system working as the main system, independent operation of the first computer system, and independent operation of the second computer system.
2. Description of the Prior Art
Conventional dual computer systems having a dual system controller therein have been based on the premise that the dual system controller can continuously operate trouble-free. Thus, the conventional systems have been placed in a very dangerous state when the dual system controller fails to properly operate due to trouble occurring therein. The malfunction of the dual system controller leads to a situation such that it detects non-coincidence between the data outputs of the first and second computer systems in spite of the fact that these two computer systems are delivering the same and correct output data. In such a case, the dual system controller detecting the non-coincidence between the data outputs applies an interruption request signal to these computer systems. In response to the application of the interruption request signal, a diagnose routine is run through these two computer systems for checking as to whether the output data are correct or not. In this case, the result of checking with this routine proves that these two computer systems are delivering the same and correct output data, and each individual computer system judges that its own system is accurately operating and cuts itself from the other system to directly apply its output data to an external output register. This is a very dangerous state for the dual computer system of this kind.